Transformer construction



y 1959 H. B. FLEMING 2,885,643

I TRANSFORMER CONSTRUCTION Filed Dec. 31, 1956 2 Sheets-Sheet 1INVENTOR. 26 Hue/1 B. FLEMING BY HT TOENEYS y 5, 1959 H. B. FLEMING2,885,643

TRANSFORMER CONSTRUCTION Filed Dec. a1, 1956 2 Sheets-Sheet A 42 k fINVENTOR. Hus/1 5 FLEM/NG hwMana /W AT TORNEYS United States PatentTRANSFORMER CONSTRUCTION Hugh B. Fleming, Burlingame, Calif. ApplicationDecember 31, 1956, Serial No. 631,909 6 Claims. (Cl. 336-96) Thisinvention relates to a method of and means for constructing miniaturetransformers.

In the art of transformer construction with the advent of ferrite coresand the need for low inductance transformers there has been anincreasing tendency towards miniaturization of transformer components.The aforesaid miniaturization has caused obvious difficulties in thefabrication of inductive equipment which has made their constructiondifficult and costly.

This invention has for its principal object method of and means fortransformer construction which greatly simplifies fabrication oftransformers and which is particularly adaptable for obtaining improvedminiaturized transformers.

In this invention there is provided a novel transformer coil form formedof a single piece of resilient dielectric material which is bent in anovel way to provide terminal blocks, a core support base and a coilform into which a core member may be inserted after the Winding has beenwound on the form.

One principal object of this invention is to provide a coil formcomprising a blank which may be formed around a mandrel and wound withselected windings. The coil and form can be removed from the mandrel andone core member can be inserted into the coil form in a positionpreviously occupied by the mandrel and another core member can fitaround the outside portion of the winding in contact with the firstcore.

Another object of this invention is to provide an exterior sleevehousing which deforms the coil and form to hold a two piece core of thetransformer in tightly compressed alignment with each other and with thetransformer windings.

A feature and advantage of this invention is that upon assembly of atransformer having a core comprising an I and a U-shaped core member thesleeve housing co-acts with the coil structure to urge the legs of theU-core tightly against the end portions of the I-shaped core member inrigid alignment so that there is obtained a fixed electricalrelationship between the two core members and the coil which ismaintained under pressure.

Another object of this invention is to provide a novel method of holdingtwo core members and a coil member of a transformer structure inrelative fixed alignment during the interval the l.' members are incasedor filled in a molten plastic substance such as epoxy resin.

Other objects of the present invention will become apparent upon readingthe following specification and referring to the accompanying drawingsin which similar characters of reference represent corresponding partsin each of the several views.

In the drawings:

Fig. l is a perspective view of an embodiment of the transformer of theinvention.

Fig. 2 is a top plan view of the blank for the coil form of thetransformer.

Fig. 3 is a top plan view of the transformer in position on a windingmandrel.

Fig. 4 is a sectional view of Fig. 3 taken at line 4-4.

Fig. 5 is a top plan showing the transformer in position to be insertedinto its sleeve housing.

Fig. 6 is an enlarged section of the completed transformer structure.

Fig. 7 is a cross-section of Fig. 6 taken at line 77.

Fig. 8 is a perspective view of the blank of Fig. 2 looped over toprovide a coil form annulus.

Referring now to the drawings it can be seen that the completedtransformer assembly A is housed in a casing, as shown by Fig. 1,similar to paper capacitors and the like. The transformer casing isformed of a tubular member forming a sleeve housing 15 with pigtailleads 16 connected to the transformer windings and projecting from eachof the two opposite ends of the sleeve. The entire unit ordinarily isconstructed small enough to be mounted directly on pluggable units orwired to tube sockets under conventional electronic mounting chassis.

The transformer assembly A comprises an I shaped ferrite core 20 and aU-shaped ferrite core 21 with one or more coils 23 disposed around theI-shaped core 20 on a coil form 25. The pigtails 16 are mounted onterminal tabs or flanges 26 of coil form 25 and are connected to therespective coils 23 by lead wires 39. The coil form 25 is formed ofresilient dielectric material shaped in a T-shaped blank with a stem 27looped over in an oval annulus to provide a form for the coil windings23 and an arm 28 projecting outwardly from two opposite sides of one endof stem 27. The stem is formed with the tab flanges 26 forming the twoopposite end edges of the arm. Apertures 33 are formed in the flanges 26to facilitate mounting of pigtails 16 thereto and the connection of leadwires 39 from windings 23 to the pigtails.

In the first steps of construction of the transformer assembly A theblank of form 25 is placed with arm 28 adjacent a fiat surface 34 of amandrel 35 which is substantially the same size and shape incross-section as the I-shaped core 20. Thereafter the stem 27 is loopedaround the mandrel 35 to form an annulus substantially circumscribingarm 28 and with said arm defining a substantially flat base piece withits opposite end extremities extending beyond the ends of the annulus.The looped over stern 27 thus defines an annular or oval coil mountingform. Various dielectrical adhesives may be applied to hold the form inposition. However, it has been found that Mylar tape with athermosetting adhesive applied thereto is particularly suitable to holdthe various components of the transformer in alignment. Mylar tape is atrade name of the Du Pont Company and is generally characterized asbeing a platsic tape with an adhesive coating on one side thereof whichis in its first state pressure sensitive, but which after being heatedcures to form a relatively infusible, hard bond between the adhesivecoated surface of the tape and the adjoining surface adhered thereto.

A coil 23 is wound on form 25 around the oval formed stem 27 by rotatingthe mandrel in a lathe. Additional windings 23 may be wound around theperiphery of stem 27 by separating the windings with a dielectric tape.The Mylar tape above described with a thermosetting adhesive has beenfound particularly useful for this purpose. Such tape with a heatsetting adhesive applied thereto facilitates binding the windings 23 inplace by heat after the coil has been wound and inserted in the sleeveas will be hereinafter described.

After completing the windings each terminal lead 39 is aligned with arespective aperture 33. Pigtails 16 are then inserted in the apertures33, cinched thereto, and soldered to the terminal leads 39.

Upon completion of the windings 23 the form 25 is removed from themandrel 35 and 1 core 20 is then placed onarm 28 .of the form within thebore defined by the looped over stem or annulus 27 of the form. The 1core 20 is longer than the width of the looped stem 27 so that the twoend portions 41 of 1 core 20 project outwardly beyond the ends of thelooped over stem.

The U core 21 is dimensioned with the inside length of its web 42approximating the width of stem 27 and the outside distance between theends of the two legs 43 of the U-core 21 equal to the length of 1 core25 so the terminal ends of leg 43 can be. placed on the top surface ofthe end portions 41 of the 1 core 20 projecting beyond stem 27. The legs43 of the U core 21 are long enough to provide a small amount ofclearance between theweb 42 and the coils 23. In this position the edgesof stem 27 abut the inside of legs 43 to hold the U core 21 from movinglongitudinally with respect to the form 25. However, if desired the Ucore can be held in place by a thermosetting adhesive to preventlongitudinal movement of the core with respect to the form.

In the order of assembly of the transformer prior to the insertion of 1core member 20 into the bore of stem 27 the inner wall of the stem iscoated with a suitable thermosetting or other kind of adhesive 45 whichis of a type that will not set-up until after the component parts havebeen properly assembled within sleeve 15 as will be describedhereinafter. The U core 21 is then placed in position over the coil 23against the 1 core 20.

In the next step of construction the entire transformer assembly A isinserted into the bore of sleeve 15 with pigtails 16 projectingoutwardly from the two opposite ends of the sleeve. The outer surface ofweb 42 is positioned adjacent one portion of the interior wall as at 15of sleeve 15 and the coils 23 and coil form 25 are arranged to abut theopposite portion of the interior wall of the sleeve as at 15 The sleeve15 is dimensioned small enough so that the coil 23 and form 25 must bedeformed in order to fit within the sleeve. The coil form 25, coil 23and the flanges 26 engage with the interior wall surfaces as at 15* ofsleeve 15 to force the Tcore 20 against the U core 21 to forcefully urgeand continually bias the connecting end portions of the two core memberstogether. In this type of transformer construction it is highlydesirable and often absolutely necessary that the adjoining edges of twocore members be urged together under pressure to obtain a properelectrical union. The. action of the coil and form against the sleeveprovides ample pressure to form a proper electrical union between thetwo cores.

After the transformer assembly A is positioned in the sleeve midwaybetween the ends, the entire unit is subjected to sufficient heat tocure the heat setting adhesive to bind the winding of the coils and lockthe I core 20 in the bore of the stem 27 against longitudinal movementtherein.

In the next step of construction the transformer is filled at both endsof sleeve 15 with an epoxy resin such as epichlorohydrin-bisphenal-Apolymers. The resin filler fills in the voids within the sleeve andseals the transformer A from Contact with the atmosphere. It has beenfoundthat it is desirable to heat set adhesives binding the core and thecoil subsequent to being inserted into sleeve 15 but prior to theapplication of the epoxy resin to allow sufiicient flexibility andresiliency of the coil form to facilitate alignment of thetransformercomponentsi'undercompression.

It is desirable to cure or set the adhesive 45 prior to the step of'filling the sleeve with epoxy resin in view of the fact that when theresin is first applied as a filler there is a tendency for it to act asa lubricant and seep between the various engaging surfaces of thecoremembers and the coil form. In prior art manufacturing techniques thelubricant effect of the liquid resin has been found tooccasionallyresult in some relative movement Uff the-z I core- Ztl withrespect to the coil'form and U r core 21 even though the two cores wereheld with the ends biased together under pressure. The heat curedadhesive holds the 1 core in place in form 25 and prevents misalignmentof the 1 core with respect to the coil form. The U core is held inalignment by its legs 43 being compressed against the edges of stem 27.Thus all three components, coil form and the two cores are held inrelatively fixed position. This is quite important in transformerconstruction where the precise alignment of core members is a criticalfactor in the proper electrical characteristics of the transformer.

ter the epoxy resin has been applied and cured the transformer iscompleted and is sealed on the sides by sleeve 15 and on the ends by theepoxy resin so that the transformer is completely isolated from fluidcommunication with the outer atmosphere. This protects it fromvariations of humidity and harmful gases and also provides a certainamount of insulation from ambient heat variations.

It can thus be seen by this invention that acomplete miniaturetransformer can economically be constructed in which the outsidecasing-i.e., sleeve ll5performs the multiple functions of holding thevarious components of the transformer under compression to forcealignment during and after the assembling of the transformer, as well asforming a protective covering for the transformer and mold for the epoxyresins.

By the steps of applying the therrno-curing adhesive during the firststep of construction and curing same during a subsequent step, that is,after the transformer unit has been inserted in sleeve 15, the maximumflexibility of components is retained until a desired alignment of partsis obtained and the cores can not slide out of alignment during thefinal step while the unit is filled with the epoxy resin material.

It is also within the broad scope of the invention that other types ofadhesive including relatively quick drying single stage adhesives may beutilized. The basic requirement of any adhesive employed in the methodof manufacturing is that the inner core and coil remain relativelymovable with one another after the adhesive is applied at least untilsuch time as the entire I core assembly is fabricated and snuglypositioned within the sleeve housing 15. After assembling the adhesivemust be capable of functioning as a bonding agent between the inner coreand coil particularly during the interval that the housing is filledwith a filler.

Although the foregoing invention has been described in some detail byway of illustration and example for purposes of clarity ofunderstanding, it is understood that certain changes and modificationsmay be practiced within the spirit of the invention as limited only bythe scope of the appended claims.

What is claimed:

1. A transformer structure comprising; a resilient annular coil form,exterior coil windings around said form, a first core member axiallydisposed within the bore of said form having end portions projectingoutwardly from the two ends of said form, a second core member mountedexteriorly of said coil and disposed with its terminal ends in contactwith opposite end portions of said first core member, a rigid sleevesnugly housing said form and core members, said sleeve dimensioned tocompress said resilient form within the bore of said sleeve urging thecontacting portions of said first and second core members tightlyagainst each other and a dielectric filler completely encasing said coremembers, coil and form in the bore of said sleeve to seal the encasedelements from atmosphere and hold them in rigid encasement in thesleeve.

2. A transformer structure comprising; an annular coil form, exteriorcoil windings around said form, a first core member axially disposedWithin the bore of said form having end portions projecting outwardlyfrom the two ends of said form, a second core member mounted exteriorlyof said coil and disposed with its terminal ends in contact withopposite end portions of said first core member, and a sleeve snuglyhousing said form and core members, said sleeve dimensioned to snuglyengage said form to maintain said form, and first and second coremembers in bias contact with one another within said sleeve.

3. A transformer structure comprising; a resilient annular coil form,exterior coil windings around said form, a resilient flat base memberdisposed within the bore of-said form having flange members projectinglaterally adjacent two opposite ends of the said base member, a firstcore member disposed within the bore of said form on said base memberand having end portions projecting outwardly from the two ends of saidform, a second core member mounted exteriorly of said form and disposedwith its terminal ends in contact with the top surface of opposite endportions of said first core member, and a rigid sleeve snugly housingsaid form and core members, said second core member and flanges of saidbase member each urged against oppositely facing sides of the interiorwall of said sleeve snugly enough to deform said base member to create acontinuous pressure urging said first and second core members toward theportion of the sleeve wall adjacent said second core member, whereby thecontacting end portions of said core members are maintained in biasedcontact with one another within said sleeve.

4. A transformer structure comprising a resilient coil form having anannulus with a substantially flat base member axially disposed withinsaid annulus and projecting outwardly from the ends of said annulus,said base member having traversely extending flanges adjacent the twoopposite ends of said base member, windings around the exterior of saidannulus, a first core member mounted on the base member axially alignedin the bore of said annulus and extending exteriorly of the ends of saidannulus but terminating inwardly of said flange members, a U-shapedsecond core member having a web adjacent the exterior of the coilwindings on said annulus and two depending arms each abutting the topsurface of two of the opposite end portions 'of said first core member,and a rigid sleeve snugly housing said form and core members, saidsecond core member engaging a first interior portion of said sleeve andsaid form and windings engaged with a second interior wall portion ofsaid sleeve oppositely facing said first interior wall portion, saidsleeve dimensioned small enough to deform said base member, coil andform to create continuous pressure urging said first and second coremembers toward each other whereby the contacting end portions of saidcore members are maintained in biased contact with one another in saidsleeve.

5. A transformer structure according to claim 4 and wherein the legs ofsaid U-shaped core member engage the end edges of said annulus to limitrelative longitudinal movement between said second core member and form.

6. A transformer structure comprising; a coil form made of sheetmaterial which in blank form defines a T-shaped section having a stemwith an arm projecting transversely from two opposite sides of one endof the stem, said stem looped to form an annulus substantiallycircumscribing said arm with said arm defining a sub stantially flatbase piece within said annulus with opposite end extremities extendingbeyond opposite ends of the annulus, coil windings around the exteriorof said annulus, a first core member having a length greater than thelength of the annulus disposed axially within the bore of the annulus onsaid base piece with opposite ends of said first core member extendingoutwardly from the both opposite ends of said annulus but terminatinginwardly of the ends of said arm, a second core member mounted adjacentthe exterior of the annulus and coil, the two opposite terminal ends ofsaid second core member in contact with opposite end side portions ofsaid first core member and a sleeve dimensioned to snugly receive andresiliently deform said annulus and base piece to cause said first andsecond core members to be biased with their contacting end portionstightly forced toward each other.

References Cited in the file of this patent UNITED STATES PATENTS2,311,450 Marsh Feb. 16, 1943 2,414,525 Hill et a1 Jan. 21, 19472,632,211 Trigg Mar. 24, 1953 2,731,607 Gould et a1 Jan. 17, 1956

